This invention relates to communication systems; and more particularly, it relates to multipoint-to-point CDMA communication systems.
As used herein, the term "multipoint-to-point" refers to a communication system in which several transmitting stations which are located at various points send data to a single receiving station which is located at another point. One way to operate such a system is to have each transmitting station send its data as an amplitude modulated, frequency modulated, or phase modulated signal in its own wireless channel which differs in frequency for each station. However, if the total number of transmitting stations in the communication system is large, then a correspondingly large number separate frequency bands is required.
Alternatively, each of the transmitting stations in the multipoint-to-point communication system can send its data over a separate cable. However, when the transmitting stations are remotely located from the receive station, too much connecting cable is required.
By comparison, with a multipoint-to-point CDMA communication system, all of the transmitting stations send their data in either a single wireless channel or a single cable. By the term "CDMA" is herein meant "code division multiple access". In a CDMA system, each transmitting station modulates the data that it sends by a respective spreading code which is unique to that station. That modulated data from all of the stations is sent on the single wireless channel/cable in one frequency band to the receiving station; and in that transmission, all the CDMA signals are added together. Then, in the receiving station, the data from any one particular transmitting station is recovered by multiplying the composite CDMA signal by the same spreading codes which that one particular transmitting station used.
One prior art CDMA communication system is described in U.S. Pat. No. 4,908,836 by Rushforth et al entitled "Method And Apparatus For Decoding Multiple Bit Sequences That Are Transmitted Simultaneously In A Single Channel". Also, another CDMA communication system is described in U.S. Pat. No. 5,031,173 by Short et al entitled "Decoder For Added Asynchronous Bit Sequences". Both of these Patents are assigned to the assignee of the present invention.
In prior art CDMA communication systems such as those cited above, the spreading codes which the transmitting stations send in their CDMA signals occur asynchronously to each other. Consequently, those codes interfere with each other by an increased amount as the number of transmitting stations increases. This in turn limits the maximum number of transmitting stations that can simultaneously send their data with an acceptable error rate to the receiving station.
Accordingly, a primary object of the present invention is to provide an improved multipoint-to-point CDMA communication system in which the interference between the spreading codes is reduced so a greater number of transmitting stations can simultaneously send data.